Method and device for determining the position of communication satellites

ABSTRACT

Described is a device for determining the attitude on board of a satellite ( 1 ) with a data transfer device for carrying out optical data exchange between at least two satellites ( 1, 3 ); a device for determining one&#39;s own attitude; a device for determining the directional vectors in an inertial co-ordinate system from one&#39;s own attitude and from attitude data of at least two satellites ( 1, 3 ), said attitude data having been transmitted by means of the data transmission device; a device for determining the directional vector directed to the satellites ( 1, 3 ) of which there are at least two, in a fixed-body co-ordinate system of the satellite; and a device for attitude determination from the directional vectors determined. Also described is a method for carrying out attitude determination.

BACKGROUND OF THE INVENTION

The invention relates to a device and a method for determining theposition of one or several communications satellites.

Determining the attitude and control, i.e. the angular attitude, whenoperating present-day satellites, requires a precision in the magnitudeof 1/100° for all three rotation axes. To meet these requirements,either star sensors or earth sensors which allow independent on-boardoperation, are available. Both measuring methods for determining theattitude of satellites are associated with the disadvantage that theyrequire very considerable functional expenditure, in particular inrelation to error detection. Star sensors which allow independenton-board operation are associated with the additional disadvantage inthat in the region of the orbit of satellites there is intensiveradiation with heavy atomic nuclei. Such radiation can only be reducedwith great technical expenditure by respective hardening of thehighly-integrated electronic components. However, there still remains aresidual irradiation which acts on the electronic components, saidradiation having a negative effect on the service life of saidelectronic components.

SUMMARY OF THE INVENTION

It is thus the object of the invention to provide a device and a methodfor determining the attitude of communications satellites which deviceand method meet the requirements for operational accuracy of attitudedetermination, and which can be realised with a minimum of technicalexpenditure, in particular by obviating the need for at least oneprecise sensor.

This object is met by the characteristics of the independent claims.Alternative embodiments are provided in the subordinate claims.

By using optical data transmission systems for determining the attitudeof the satellites, said optical data transmission being necessary anywayfor the operation of communications satellites, the device and themethod of the invention require relatively little technical expenditurebecause the data required is already available on board the satelliteand can be processed using relatively simple methods.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

Below, the task is described by means of the enclosed figures. asfollows:

FIG. 1 shows a constellation of three satellites on the same orbit, saidsatellites exchanging data among themselves for attitude determinationaccording to the invention; and

FIG. 2 is a block diagram of the functions necessary for setting andcontrolling an axis of the optical system, said functions being used toset and receive a laser beam from another satellite, said laser beambeing intended for the exchange of useful data. From these data, themeasured variables are determined which, apart from the attitude data ofthe satellites involved, are required as input quantities by theattitude determination system according to the invention.

DETAILED DESCRIPTION

FIG. 1 shows three satellites 1, 2, 3. The attitude determination systemis described starting with the satellite 2 which is located in the areabetween satellites 1 and 3. The satellites 1, 2, 3 are in an orbit 5around the earth, with the centre of the earth diagrammaticallyindicated by reference number 6.

Each satellite 1, 2, 3 is able to exchange information with any of theother satellites in the constellation shown. Preferably, thisinformation exchange takes place via optical data connections by meansof laser technology. Each satellite 1, 2, 3 is able to determine itsattitude relative to the centre 6 of the earth, for example its attitudein an inertial geocentric co-ordinate system whose source point issituated in the centre 6 of the earth. By way of the optical dataconnections, each satellite 1, 2, 3 is able to transmit its own attitudeto other satellites. Conversely, there is the option that each satellite2 receives the attitude data of the other satellites 1, 3 which areinvolved in the data exchange. For example, satellite 2 is able totransmit its own attitude, in FIG. 1 indicated by vector 12, to theother satellites 1 and 3. Furthermore, by way of the optical dataconnections which exist between satellites 1 and 2 or 2 and 3respectively, satellite 2 receives the attitude data of satellite 1,said data being indicated by vector 11, as well as attitude data ofsatellite 3, indicated by attitude vector 13.

Preferably, altitude determination takes place by means of the GlobalPositioning System (GPS). For this purpose, each satellite 1, 2, 3 has arespective GPS receiver (not shown) and an associated data processingdevice for determining the attitude from sensor data on board. However,determination of a satellite's own attitude can also be undertaken usingother methods, e.g. using the estimation method by means of orbitpropagation.

Since the satellite 2 knows the attitude vectors 11, 12, 13, it is ableto determine the directional vectors to the satellites which areinvolved in the optical data connections, i.e. satellites 1, 3. Thistakes place on the basis of known mathematical operations which arecarried out by respective data processing devices provided in satellite2. In particular, satellite 2, using attitude vectors 11, 12 or 12, 13respectively, calculates the directional vector 15 which is directedfrom satellite 2 to satellite 1, and calculates the directional vector16 which is directed from satellite 2 to satellite 3, with thedirectional vectors 15, 16 preferably being determined in the inertialgeocentric co-ordinate system.

The attitude data of the transmitting satellite are preferablytransmitted via the optical data connection together with the usefuldata.

To establish optical data connections, each of the satellites which isinvolved in data exchange, comprises at least one optical system 21 withwhich the laser beams transmitted by the other satellites involved inthe data exchange, can be received and with which optical system 21,laser beams can be transmitted to the other satellites which areinvolved. The optical system comprises in particular at least onemovable telescope and several movable mirrors as well as fixed opticalelements. By means of the optical system 21, the transmitted or receivedlaser beam is subjected to optical imaging (FIG. 2) with the opticalimage being acquired or measured by means of an optical tracking sensor22. The optical sensor 22 acquires in particular the deviation 24 of thereceived laser beam, from a reference point on the imaging plane of theoptical system. By means of the optical system, which can be adjusted ina predetermined way, the received laser beam can be moved to thereference point or at least to a defined region of the reference point.On the basis of the measured deviation 24, or on the basis of anydeviation 24 still present after adjustment of the movable parts of theoptical system, together with the adjustment angle 25 at the time, ofthe movable components of the optical system, by means of known methodsa directional unit vector from the receiving satellite 2 to therespective transmitting satellite 1 or 3, can be determined. Preferably,this unit vector is first determined in the co-ordinate system of theoptical system and then by means of usual mathematical transformationsin the fixed-body co-ordinate system of satellite 2. Attitudedetermination then takes place on the basis of known mathematicalmethods, not only based on the directional unit vectors 15 or 16determined in the inertial geocentric co-ordinate system, but also basedon the directional unit vectors in the fixed-body satellite co-ordinatesystem 2, said directional unit vectors having been determined from themeasured deviations 24 and from the adjustment angles 25 at the time.

Below, by means of FIG. 2, the determination of the directional unitvectors is described, which determination takes place in a fixed-bodyco-ordinate system of the satellite, said directional unit vectors beingformed from the deviation 24 and the adjustment angle 25 at the time.For the sake of clarity, FIG. 2 shows the functional context of themechanisms associated with the determination of these variables, only inone axis. The representation in FIG. 2 analogously applies to threeaxes.

The optical system 21 by which imaging of a received laser beam takesplace, can be adjusted by means of a coarse pointing assembly (CPA) 30and a fine pointing assembly (FPA) 50. From the point of view of devicetechnology, preferably the telescope of the optical system 21 is held onthe CPA, while the movable mirrors are preferably located within the CPAand held on the FPA. The fine pointing assembly (FPA) 50 is thussituated functionally downstream of the coarse pointing assembly (CPA)30. Usually the coarse pointing mechanism 30 comprises a two-axisrotation mechanism 31 with which the azimuth angles and elevation anglesof the telescope can be set; drive electronics 32 for the two-axisrotation mechanism 31; and CPA sensors 33 for measuring the azimuthangle and elevation angle at the time, of the rotation mechanism 31. Arespective CPA controller 34 is arranged functionally upstream of thedrive electronics 32, said CPA controller determining desired signals 35for the CPA drive electronics 32. The CPA drive electronics 32 in turnissues respective corrective signals 36 to the two-axis CPA rotationmechanism 31. The CPA sensor 33 which can comprise a group of sensors,determines the measured rotation angle 38 from the respective actualrotation angle 37 of the CPA rotation mechanism 31. By way of an actualrotation angle 37 of the telescope, a received laser beam 40 issubjected to a change in angle, with the size of the change depending onthe rotation angles and the optical imaging characteristics of thetelescope 39. This results in an angle of incidence 43 of the laser beam40.

The fine pointing assembly 50 also comprises a two-axis rotationmechanism 51; FPA drive electronics 52 arranged upstream; and an FPAregulator 54 arranged upstream of said FPA drive electronics 52. The FPAregulator 54 transmits a desired FPA signal 55 to the FPA driveelectronics 52 which sends an FPA corrective signal 56 to the FPArotation mechanism 51. The actual rotation angle 57 measured at thetime, of the FPA rotation mechanism 51, is measured by an FPA sensor 53or an FPA sensor system 53. The measured rotation angle or rotationangles 58 are available for further signal processing or data processingin processor 59. The settings of the movable mirrors, said settingsbeing carried out by the fine pointing assembly 50, result in opticalimaging 59 and thus a change in angle 63 which is smaller than thechange of angle 43.

By way of the summation location 70, FIG. 2 shows that the change inangle of incidence 43 due to the coarse pointing assembly 30 and thechange in angle of incidence 63 due to the fine pointing assembly 50,add up. Due to this change in angle 43, 63, an optical angle 71 resultswhich is imaged by optical imaging by means of the optical system 21.Optical imaging by means of the optical system 21 results in an actualdeviation 73 from the reference point of the imaging plane of theoptical image of the optical system 21. As has already been described,the optical sensor 22 determines a measured deviation 24 accordingly.

The deviation 24 measured by the optical sensor 22 is fed to the FPAregulator 54. Furthermore, the rotation angle 58 measured by the FPAsensor 53 is fed to the FPA regulator 54. The measured rotation angle 58also continues to be fed to the CPA regulator 34. Based on the rotationangle 38 measured by the CPA sensor 33 and the rotation angle 58measured by the FPA sensor 53, the CPA regulator 34 is able at any time,to determine the desired signal 35 for the CPA drive electronics 32.Analogously, from the measured rotation angle 58 and the measureddeviation 24, the FPA regulator 54 is able to determine the desired FPAsignal 55.

The adjustment angles 25 at the time, i.e. the measured rotation angles38, 58 and the measured deviation 24 are fed to a calculation unit 80.In this calculation unit 80 these currently received input data 24, 25are converted in the fixed-body co-ordinate system of the receivingsatellites 2, to become the directional vector which is directed fromthe receiving satellite 2 to one of the neighbouring satellites 1, 3 orto another satellite involved in data exchange.

The representation of FIG. 2 only refers to one adjustment axisrespectively, of the coarse pointing assembly 30 and the fine pointingassembly 50. Preferably however, adjustment of both the coarse pointingassembly 30 and the fine pointing assembly 50 takes place in two axes.With each change in attitude of the satellite 2, i.e. with each rotationof the satellite on one or several spatial axes, by means of theadjustment angles 25, the laser beam 40 to be received for establishingan optical data connection to other satellites, is held in all fouradjustment axes in a reference point of the imaging plane via theoptical system 21. Expansion of the representation shown in FIG. 2referring only to one axis, to all spatial axes and the four adjustmentaxes, takes place according to known control-technological andmathematical considerations.

Thus in relation to two satellites which are exchanging data, e.g. inrelation to satellites 2 and 1, emanating from satellite 2, two vectorsare available for determining the attitude of satellite 2: firstly thedirectional vector 15 in the inertial geocentric co-ordinate systemwhich vector is arrived at from determining the attitude and the mutualtransmission of the attitude data, and secondly the vector directed fromthe receiving satellite 2 onto the other satellites exchanging data withsaid receiving satellite 2, e.g. satellite 1 in the fixed-bodyco-ordinate system of the receiving satellite 2 which has beendetermined in the calculation unit 80.

Attitude determination according to the invention requires these twovectors in relation to at least two satellites 1, 3 which are exchangingdata with the receiving satellite 2. Preferably this data exchange isachieved by means of optical data connections, but it can also takeplace in any other way. The data exchange necessary for attitudedetermination is preferably carried out as part of the exchange ofuseful data which has to be carried out anyway. Consequently, theattitude determination method according to the invention results in onlyvery slight additional technical expenditure. At least two satellites 1,3 which together with their laser beams and the attitude information,transmitted via said laser beams, for determining the attitude ofsatellite 2, are preferably on the same orbit as the receiving satellite2. They can however also be situated on different orbits; this is of noconsequence for the method according to the invention. Preferably suchneighbouring satellites are selected whose angle to satellite 2 betweenthe directional vectors 15, 16 is as large as possible. Preferably morethan two transmitting satellites which are partly on different orbits,are used to determine the attitude of the satellite 2 which is receivingat the time. The more satellites 1, 3, transmitting at the time, thatare available, the more the accuracy of the method can be improved. Thesatellites 1, 3 transmitting at the time can at other times orsimultaneously be receiving satellites so as to determine their ownattitude from the data received.

From the minimum of four directional vectors, i.e. two directionalvectors in the inertial geocentric co-ordinate system and twodirectional vectors in the fixed-body co-ordinate system of thesatellite, the satellite determines its attitude with the use of knownmathematical methods. Preferably, calculation of the attitude of thesatellite 2 which is receiving at the time, is carried out according tothe following method: from the two directional vectors 15, 16 anorthogonal 3 by 3 matrix is formed by means of two intersectionproducts. This is carried out in the same way for the two sets ofdirectional vectors which are given in the inertial geocentricco-ordinate system and in the fixed-body co-ordinate system of thesatellite. Two matrices are thus obtained which describe the sameattitude in the two co-ordinate systems. By multiplying the matrixformed from the directional vectors in the inertial co-ordinate systemby the resolvent of the matrix formed from the directional vectors inthe fixed-body co-ordinate system of satellite 2, the required attitudematrix of satellite 2 results.

In the case where the satellite 2 which is receiving at the time, iscommunicating with its neighbouring satellites 1, 3, calculation of theattitude is unequivocal, provided the three satellites 1, 2, 3 are notattitudeed in a straight line, a configuration which can be excluded forsatellites in a constellation with the same orbit or orbit altitude.

If more than two neighbouring satellites are available, calculation ofthe attitude of the satellite is overdetermined. In this case theaccuracy of attitude determination can be improved, due to theadditional measurement information, by using calculation methods foroverdetermined equation methods, in particular least-squares methods orminimum-variance estimates. Furthermore, dynamic estimation methods,e.g. based on sequential processing of the measured data, in particularby means of Kalmann filters, can also be used.

The measuring errors of the sensors contain high-frequency fractions,e.g. a frequency above 20 Hz, which are for example caused by noise orinterference emanating from the stabilising flywheels used forstabilising the satellites. Determining the attitude of the satelliterequires attitude readouts at a significantly lower frequency.High-frequency fractions in the attitude readout can therefore befiltered out by a respective filter, in particular a low-pass filter ora Kalmann filter. This will improve the attitude readouts. Furthermore,mechanical attenuation measures and/or further precautions such as notchfilters or an active control system, can be provided.

Pointing errors during integration of the satellites as well as thermaldeformation cause static errors or errors which change only slowly, inthe pointing of the payload. Pointing of the payload can be improved inthat a relative attitude measuring sensor is fitted between the opticalcommunication terminals and the payload (e.g. the antennae). Themeasured relative attitude is then used to correct pointing of thepayload accordingly.

Thus, the method according to the invention comprises the steps:calculation of two vectors in the inertial geocentric co-ordinate systemin the direction of at least two neighbouring satellites, based onmeasuring the attitudes, for example by GPS and transmission of theattitude data by means of the already existing optical data exchange;calculation of two vectors in the fixed-body co-ordinate system of thereceiving satellites (said vectors corresponding to the direction of theincoming laser beam from the satellites exchanging data with thereceiving satellites, e.g. the two neighbouring satellites) based on thesensor information of the optical system, i.e. the angle attitude of thecoarse pointing assembly 30, the angle attitude of the fine pointingassembly 50, the measured deviation 24 of the optical sensor 22;calculation of the satellite attitude from at least two sets of twodirectional unit vectors each, which have been determined in theinertial geocentric co-ordinate system and in the fixed-body co-ordinatesystem of the satellite; filtering of the high-frequency fractions ofthe measured value disturbances; and if necessary improvement in thedetermination of the payload attitude by relative attitude measuringbetween the payload and the optical communication terminal.

What is claimed is:
 1. A device for determining the attitude on board afirst satellite comprising: a data transfer device for carrying outoptical data exchange between said first satellite and at least twoother satellites; a device for determining attitude of said firstsatellite; a device for determining directional vectors in an inertialco-ordinate system based on the attitude of said first satellite and ondirectional vectors from said at least two other satellites, saiddirectional vectors being adapted for transmission by the data transferdevice between said first satellite and said at least two othersatellites; a device for determining a respective directional vectordirected to said at least two other satellites, in a body-fixedco-ordinate system of said first satellite; and said device fordetermining in said first satellite a respective directional vector fromsaid at least two other satellites including means for receiving laserbeam signals of said directional vectors from said at least two othersatellites, a coarse pointing assembly and a fine pointing assembly,said coarse pointing assembly and said fine pointing assembly beingconnected to correct any deviation of said laser beam signals of saiddirectional vectors with respect to a reference.
 2. The device of claim1, wherein the device for determining said respective directional vectorin the body-fixed co-ordinate system of the first satellite furtherincludes: an optical system for optical imaging of the laser beamsreceived by said at least two other satellites, said coarse and finepointing assemblies being connected to the optical system for adjustingthe optical system in an imaging plane in the region of said referenceduring a change in attitude of the first satellite; a measuring devicefor measuring adjustment angles of the coarse and fine pointingassembles, an optical sensor for measuring deviation of the receivedlaser beam signals with respect to the reference in the imaging plane; acalculation unit which receives the deviation and the adjustment anglesand determines a respective directional vector in the fixed-bodyco-ordinate system of said first satellite with respect to said at leasttwo other satellites engaged in optical data exchange with said firstsatellite.
 3. The device of claim 2, wherein said coarse pointingassembly has an output representing said deviation which is combinedwith the laser beam signal from the respective of said at least twoother satellites, said fine pointing assembly having an output which isadded to the combined signal of the output of the coarse pointing systemand said laser beam signal to produce a resultant signal.
 4. The deviceof claim 3, wherein the resultant signal is connected to said opticalsystem.
 5. The device of claim 1, wherein attitude determination of thesatellites takes place by means of a GPS system.
 6. The device of claim1, wherein attitude determination takes place by means of orbitpropagation.
 7. The device of claim 6, wherein with attitudedetermination by means of orbit propagation, support for measured datais additionally provided.
 8. The device of claim 1, wherein the firstsatellite receives the laser beam signals for more than two of saidother satellites involved in data exchange with said first satellite. 9.The device of claim 1, wherein for determination of the directionalvectors, calculation methods for overdetermined equation systems areused.
 10. The device of claim 9, wherein least-square filters are used.11. The device of claim 1, wherein for determination of the directionalvectors, sequential processing of the measured data takes place.
 12. Thedevice of claim 11, wherein Kalmann filters are used.
 13. The device ofclaim 1, further including mechanical attenuation devices and dynamicfilters are provided to reduce disturbance as a result of satellitevibration.
 14. The device of claim 1, wherein a relative attitude sensoris provided between an optical communication terminal and a payload ofthe satellite to improve attitude information of the payload.
 15. Amethod for determining the attitude of a first satellite, comprising thefollowing steps: calculating a directional vector in an inertialgeocentric co-ordinate system of said first satellite in the directionof at least two other satellites involved in data exchange with saidfirst satellite based on measuring attitudes of the satellites involvedin the data exchange, and on transmitting the attitude data by means ofoptical communication; calculating a directional vector in a body-fixedco-ordinate system of said first satellite which directional vector isdirected from said first satellite to said at least two other satellitesinvolved in data exchange with said first satellite, based on coarse andfine evaluation of sensor information regarding setting of an opticaldevice receiving a laser beam signal from a respective satellite of saidat least two other satellites involved in data exchange and based ondeviation of said optical laser beam from a reference point in a fixedimaging plane in said body-fixed co-ordinate system; and calculating theattitude of the first satellite from at least two directional vectors inthe inertial geocentric co-ordinate system and the body-fixedco-ordinate system of the first satellite.